Prior art flywheel energy storage systems have been designed primarily for stationary use, due to their bulk and their vulnerability to vibration.
Known mobile systems for storing electrical energy, such as batteries, have had high mass and limited capacity. However, for motor vehicle applications, where acceleration calls for substantial energy inputs, and recuperative braking can generate comparably large amounts of energy to be stored, it would be desirable to have a relatively low-mass, high-storage-capacity system adapted for mobile use. A suitably designed flywheel system could satisfy these needs.
Also, the telephone industry has long used lead acid batteries for back-up power to provide uninterruptible service. The typical telephone network sends signals over optical fiber from the central office to a remote terminal. Each remote terminal supports approximately 1000 homes. The cable companies use a similar configuration, where signals are sent from the "head end" (cable company office) to remote terminals servicing approximately 500 homes. In both cases the remote terminal uses power provided by the local utility to carry the signal from the terminal to the subscriber, since fiber optic cable cannot carry electricity. To support the terminal during a utility outage, the phone or cable companies install a back-up power supply, typically an uninterruptible power supply that uses batteries as a power source.
It is desirable to eliminate batteries from these networks because of their limited life, poor reliability, and high maintenance requirements. One replacement for batteries is the flywheel energy storage system.
U.S. Pat. No. 4,211,452 describes an inertia wheel more particularly adapted to space applications. It includes the combination of a peripheral type of motor with permanent magnet on the rotor and ironless winding on the stator. This structure limits speed due to stress. The current of the winding is switched electronically by an amplitude modulation system, associated to a reactance coefficient varying circuit, and reversal of direction of rotation of which is achieved by permutation of the control circuits. There are also provided bearings formed by a passive radial magnetic centering device and a redundant active axial magnetic centering device slaved to an axial rate detector. This device requires a permanent magnet and four control coils just for axial control.
U.S. Pat. No. 4,620,752 describes a magnetic bearing having position stabilization of a supported body which includes damping and aligning arrangement. An application of the magnetic bearing is illustrated showing a magnetic bearing system for a flywheel. This system requires combining two control coils with two rotating permanent magnets for each bearing.
It can be appreciated that new and improved flywheel systems are desired, in particular, for mobile use and for back-up power supply systems to provide uninterruptible power supplies.